Add some experimental module interfaces

...with somewhat placeholder names as they're the best I can think of right now:

 * 'Extras': To add new bits of UI to places (specifically the space panel)
 * 'Builtins': For modules to render components that are part of Element Web
 * 'Navigation': For modules to add paths to the URL router

packages/element-web-module-api/src/api/extras.ts

@@ -0,0 +1,18 @@
+/*
+Copyright 2025 New Vector Ltd.
+
+SPDX-License-Identifier: AGPL-3.0-only OR LicenseRef-Element-Commercial
+Please see LICENSE files in the repository root for full details.
+*/
+
+import { JSX } from "react";
+
+interface SpacePanelItemProps {
+    isPanelCollapsed: boolean;
+}
+
+export type SpacePanelItemRenderFunction = (props: SpacePanelItemProps) => JSX.Element;
+
+export interface ExtrasApi {
+    addSpacePanelItem(renderer: SpacePanelItemRenderFunction): void;
+}